This invention relates generally to recording devices. Read heads are commonly used in sub-systems that retrieve data from storage media in high-performance data storage devices. For example, magneto-resistive (MR) heads (or transducers) are used to read data from magnetic storage media, such as hard disk drives, tape drives, and so forth. The data are stored on the media as a series of changes in magnetic flux, and the MR head reads the data by sensing the magnetic flux and producing a differential voltage signal, also called a response signal, that includes a corresponding series of peaks with polarities that are determined by the direction of the magnetic flux. The MR head then transfers the response signal to a read channel, which reproduces the data.
The MR heads often produce asymmetric response signals. The asymmetry, which is typically caused by characteristics (magnetic nonlinearity) of the read head (and may thus be referred to as “head asymmetry”), may be defined as the relative difference between the amplitudes of positive and negative peaks in the response signal (e.g., normalized by their average value). In an ideal situation (symmetric), both the positive and negative pulses of the response signal should have the same amplitude. In the case of asymmetry, however, positive and negative pulses of the response signal differ in amplitude, and in some cases by a large amount. Typical values for the asymmetry range from 10% to 20%, although higher values (up to 30%) can be measured.
The head asymmetry impacts the performance of the read subsystem in recording systems (e.g., disc drives), which in effect impacts overall field reliability. For instance, erroneous signal detection may occur, resulting in lost data or data that is difficult to retrieve (e.g., through error correction). In conventional data retrieval systems, the asymmetry of the response signal may be compensated for by setting different thresholds for positive and negative pulses, typically accomplished during the manufacturing process after a response signal has been sampled and digitized. For example, if there is head asymmetry of 10-20% in the system, such asymmetry may be corrected through calibration. (Generally, if the head asymmetry is greater than 30-40%, the system may not be able to account for such drastic asymmetry in read pulse amplitudes.)
Notably, the calibration during the manufacturing process compensates for the head asymmetry that is based on the initial physical characteristics of the read head. However, a problem that arises after the calibration is that once the read subsystem leaves the manufacturing process, changes in asymmetry due to aging of the device (or other factors) over time are no longer compensated for, and the changes may thus impact the performance of the read-write subsystem, and may ultimately result in failure of the system (e.g., partial or full failure), caused by an inability to properly distinguish and/or interpret signals.